Segments of non-metallic conduits, such as pipes, fittings and connectors which are designed to be interfitting, are commonly joined together by inserting an end of one into an end of the other and sealing the junction with an adhesive. For example, segments of polyvinylchloride (PVC), chlorinated polyvinylchloride (CPVC) or acrylonitrile-butadiene-styrene (ABS) pipe may be joined with corresponding connectors or related fittings and the connection sealed with a solvent adhesive.
Solvent adhesives are typically liquid solvents containing a small amount of dissolved polymer resin that is the same polymer, or a chemically similar polymer, that comprises the objects to be joined. When such a solvent adhesive comes in contact with the surfaces of the objects, the solvent partially dissolves and softens the outer layers of the mated surfaces. The resin in the solvent solution fills any holes or spaces between the two surfaces by bonding to the partially dissolved surfaces and entangling the polymer chains. When the solvent evaporates, the polymer solidifies and a mechanical bond is created between the surfaces, forming a "solvent weld".
Conventional solvent adhesives for PVC, CPVC and ABS and other plastics are known in the art and generally are a mixture of solvents selected on the basis of their ability to dissolve high and/or low molecular weight resin and their different evaporation rates. A proper mixture of solvents will completely dissolve the resin in the adhesive and reduce the solution to the desired solids concentration and viscosity for application and workability. Some organic compounds in the solvent adhesive may not be true solvents for the resin, but rather function as plasticizers, softening or swelling polymer surfaces, to provide lubrication for the fitting together of conduits, such as pipe joints, and mobility for achieving proper alignment. In the context of this invention, the term "solvent" is intended to include both solvents and plasticizers for the polymer. Solvent adhesives may contain other additives, such as a colorant to make the adhesive more visible, and inert fillers and other compounds to increase the solids content and/or to control viscosity.
Conventional solvent adhesives may contain one or a mixture of several different organic solvents. For example, solvent adhesives for PVC or CPVC may contain, in addition to PVC or CPVC resin, the solvents tetrahydrofuran (THF) and cyclohexanone or a mixture of THF, methylethylketone (MEK), cyclohexanone and acetone. Similarly, a solvent for ABS in solvent adhesives is typically MEK. Other reported solvents used in solvent adhesives for PVC, ABS or CPVC are ethyl acetate, dimethylformamide and N-methyl-2-pyrrolidone. Because these solvents have different evaporation rates, the quantity of each of the solvents in the mixture determines the open time available for workability, fitting and adjustment. The cure of the solvent weld occurs when most of the applied solvents have evaporated and the solvent welded joints are fused together. Preferably, cure takes place within a reasonable period of time without the use of heat, pressure, UV light or extraordinary mechanical devices.
There are several problems associated with the use of conventional solvent adhesives. For example, the solvent adhesives are liquids and are usually applied in an open environment to plastic surfaces with a brush or other tool immediately prior to the joining of the surfaces. This application can be time-consuming and wasteful, as the liquid adhesive may spread along a greater surface area than actually desired or may drip or spill. In addition, a loss of solvents may occur by evaporation during application. The solvents themselves are flammable and many have a high volatile organic content (VOC), with resulting safety and environmental concerns.
One approach to solving the above problems, disclosed by the present invention, is to provide to the end user a suitable conduit segment, such as a pipe segment, fitting or connector, that is pre-coated with a microencapsulated solvent adhesive in a dry form. The user then simply couples the supplied conduit segment to a complementary interfitting conduit segment and, by applying ordinary mechanical force, ruptures the microcapsules to release the solvent adhesive and cement the segments together. Thus, the previous problems of unwanted waste, expense, and issues of safety and the environment are materially reduced.
It has been suggested previously that a conventional solvent adhesive, such as those described above, may be microencapsulated and coated onto a surface of a pipe. However, no disclosure of a conventional solvent adhesive that is microencapsulatable has been forthcoming. Microencapsulation of separate components of certain adhesive components has been reported. For example, pipe sections, such as polyester fiberglass or epoxy fiberglass may be joined by an adhesive composition formed by a polymerization reaction that occurs as the pipes are joined. In this system, a polymerization activator may be encapsulated in a rupturable microsphere and dispersed in a solution containing polymerizable monomers and a polymerization catalyst. The microspheres are ruptured either before or after the adhesive is applied to the pipe, releasing the activator to initiate the polymerization reaction. It is also known that common epoxy adhesives may be formed in a two-part system by separately microencapsulating the epoxy resin and the curing agent and rupturing the microcapsules to mix the two components. Solvents for use in cosmetic compositions may also be separately microencapsulated and a mixture of these capsules introduced, for example, into a suitable cosmetic vehicle before rupture of the capsules to mix the solvents. Each of the above-described systems, however, involves separate encapsulation of individual system components.
Thus, there is a need for a microencapsulatable solvent adhesive composition for plastic conduits, such as those manufactured from PVC, CPVC, ABS, or mixtures of these, that contains polymer resin and a mixture of appropriate solvents. Preferably, the solvents in the composition are aggressive in dissolving or softening the polymer in order to provide sufficient lubrication to facilitate the insertion of a conduit segment into the mating conduit segment, such as in an interference fit, as well as to improve the open time for workability and adjustment. There is a further need for microcapsules that contain a sufficient volume of a solvent adhesive composition to enhance lubrication and to cement the mated surfaces together with a desired bond strength.